After yesterday's upgrade, my site is taking 50 seconds to open (and then behaves normally)

[Ed_S]

Perhaps try

@Ed_S

my list of standard instant diagnostics:

cat /etc/lsb-release
uptime
df -h /
free
vmstat 5 5
dmesg|egrep -i "memory|oom|kill"
ps auxrc

But run these (at the command line on the machine which runs the server) at the same time as browsing the site to provoke the slowdown.